It looks like you already know a lot more than me
I'm assuming you want send and receive sms, rather than just send?
Localphone can send, and receive, but it seems to be that receiving only works on UK incoming numbers and for from UK mobile networks. I have an account, but I haven't explored this at all. Edit: now tested by receiving one.
Global SMS Support | Localphone
Some of those global SIMs have call diversion at reasonable rates, including free, but there's the cost of calling the number in the first place.